/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlinePartyInterface.h
Request to join an existing party
void RequestToJoinParty ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & LocalUserId, const [FOnlinePartyTypeId](API\Plugins\OnlineSubsystem\FOnlinePartyTypeId) PartyTypeId, const [FPartyInvitationRecipient](API\Plugins\OnlineSubsystem\Interfaces\FPartyInvitationRecipient) & Recipient, const [FOnRequestToJoinPartyComplete](API\Plugins\OnlineSubsystem\Interfaces\FOnRequestToJoinPartyComplete) & Delegate )